Method of purifying lead, tin, and alloys thereof



Patented Apr. 22,1940

STATES PATENT QFFlfiE METHOD F PURIFYING LEAD, TIN, ALLOYS THEREOF No Drawing. ,Application July is, less,

- Serial No. 218,952

2 Claims.

The present invention relates to a method for purifying metals. Its object is the purification of such metals as lead, tin or alloys of either of these metals, particularly alloys of lead and tin.

My invention more particularly deals with a simple, economical and emcient method for the removal of cadmium from lead, tin and alloys of the same.

I have found that by treating molten lead, tin or alloys of either of these metals as hereinafter described with a lead halide, e. g., lead chloride (PbClz), the metal will be purified cadmium as well as several other elements, the removal of which is described in my copending application, Ser. No. 286,565,'flled July 26, 1939. My discovery is a general one; thus all such elements to be removed are converted in the process of my invention into the respective halide and lead as metal is added to the alloy. It is at once obvious,

therefore, that my invention is particularly adaptable to the purification of lead and alloys of lead because the addition of lead does not constitute a contamination of the metal or alloys being treated.

While, as aforesaid, my invention in its broad aspects embraces the use of any lead halide or mixture of two or more such compounds, for the sake of simplicity, my invention will be particularly described with respect to the use of lead chloride. It will, of course, be understood by those skilled in this art that the conditions such as temperature at which the halide treatment is carried out may have to be varied, i. e., raised or lowered somewhat depending upon whether the lead halide has a melting point above or below that of lead chloride.

My invention may very easily be carried out in any suitable type of apparatus. The ordinary lead melting pot equipped with means for agitation is most convenient. In-carrying out the invention, the metal to be.. purified is placed in the reaction vessel and heated to above its melting point. Lead chloride is then added to the molten metal while agitating the molten mass. Reaction at once takes place between the lead chloride and the cadmium which it is desired to remove with the formation cadmium chloride. The temperature at which the process is to be carried out should preferably be above the melting point of the lead chloride. After the reaction has ceased, the purified metal may be separated from the resulting slag in any convenient manner as, for example, by skimming 01f the chloride slag.

If desired, certain other salts maybe mixed with the lead chloride for the purpose of reducing the melting point. Generally speaking, the addi tion of any salt will tend to depress the melting point of lead chloride. The halides of the alkali metals e. g., potassium and sodium chloride, are particularly efiective. For instance, the admix- 5 ture of a sumcient amount of sodium chloride to form the eutectic mixture of about 92% lead chloride and 8% sodium chloride will give a melting point of 410 C. instead of about 501 C. for pure lead chloride. In selecting the salt for low- 10 ering the melting point of the lead chloride in the practice of my invention it is necessary that the salt selected be non-reactive both with respect to the metal to be purified as well as to the lead chloride. Thereafter, I have chosen to desigl5 nate such salts as non-reactive melting point depressing salts. These will generally be used in lesser amounts than the lead chloride.

The amount of lead chloride to be employed will depend upon the amount of cadmium to be removed. Although the purification of the metal will go to substantial completion by using the stoichiometric quantity it is desirable to employan excess over this quantity in order to insure a complete removal of the cadmium. The removal of the cadmium is facilitated by stirring the lead chloride and the molten metal; the time of stirring may vary according to the quantity of the metal treated and the cadmium to be removed. The time required for the reaction cannot be definitely given for. every case but those skilled in this art will be enabled, from the description given here, to carry out my invention to effect a substantially complete removal of cadmium. As an illustrative example of the application of my process, I give the following:

I melted in a laboratory melting pot 888.5 grams of a solder consisting essentially of 50% lead and 50% tin but containing also about 8.0 grams of cadmium which constituted about 0.90 percent 40 of the alloy. Theoretically, 8 grams of cadmium would require about 20 grams lead chloride according to the equation: Cd+PbC12- CdCl2+Pb, but I stirred into the molten mass which was at a low red heat, about 40 grams lead chloride and continued stirring for several minutes. There after, I poured a sample test bar. Analysis showed that cadmium had been reduced from 0.90% to0.27%, indicating a removal of about 70% of the cadmium. Analysis of the chloride slag showed that only approximately one-third of the lead chloride added had been used up.

I have found that the above reaction has a tendency to be reversible. However, when cadmium is present in the metal to be purified sevu eral treatments according to my present invention will accomplish the substantially complete removal of cadmium, if desired.

The foregoing description of my present invention has been given for cleamess of understanding and no undue limitations should be deduced therefrom but the appended claims should be construed as broadly as possible in view of the prior art.

I claim:

1. A method of removing cadmium from lead, tin and alloys thereof which comprises melting a metal selected from the group consisting of lead, tin and alloys thereof contaminated with cadmium, admixing therewith a stoichiometric amount of lead chloride calculated upon the amount of impurity present in the metal and agitating the mass at a temperatiure above the melting point of lead chloride until the cadmium is partially removed from the molten massas a cadmium chloride slag, separating the partially purified metal from the said cadmium slag and repeating the lead chloride. treatment until the cadmium is substantially completely removed from the molten mass and separating the purified sired removal of the cadmium from the molten mass is effected by the formation of a cadmium halide slag, and separating the purified metal from the said cadmium halide slag.

GUSTAVE E. BEHR. 

